1. Field of the Invention
This invention relates to a solder resist composition and a printed circuit board, and more particularly to a solder resist composition capable being easily applied by a roller coater and having less lead migration as well as a printed circuit board using such a solder resist composition.
2. Description of Background Art
A solder resist layer is formed on an outermost surface layer portion of a printed circuit board. This solder resist layer possesses a function of protecting a conductor circuit exposed at the surface layer portion and a function as a dam for preventing solder flowing and solder bridge of a solder body (for example, solder bump) supplied onto surfaces of pads to be mounted with an electronic component.
As a resin composition for the formation of the solder resist layer, a solder resist composition obtained by dissolving epoxy acrylate and an imidazole curing agent in cellosolve acetate and adjusting a viscosity thereof to 0.1-0.2 Pa.multidot.s, and the like are used as disclosed, for example, in JP-A-63-286841 (U.S. Pat. No. 4,902,726).
Furthermore, an alkali-developing type solder resist composition is disclosed in JP-A-62-23036.
However, when the resin composition disclosed in JP-A-63-286841 is used as a solder resist layer, lead ion diffuses from the solder body (solder bump or the Like) formed on the pad into the solder resist layer (this phenomenon is called lead migration) and hence there is a problem of causing electrical conduction between the pads to form a short-circuit. Furthermore, when the resin composition disclosed in JP-A-62-23036 is used as a solder resist layer, the above phenomenon is also caused to form a short-circuit.
On the other hand, when the above resin composition is applied to a copper pattern and then dried, the copper pattern under the resin layer is oxidized. If nickel-gold plating is carried out, the oxidized layer of the copper pattern is dissolved to bring about discoloration known as the hallow phenomenon.
And also, when the above resin composition is used as the solder resist layer, there is a problem of easily peeling the solder resist layer due to heat cycle.
Moreover, a basic specification of the printed circuit board is double-sided wiring board, so that the solder resist composition should be applied onto both surfaces of the wiring board. For this purpose, when a method of sandwiching the wiring substrate between a pair of rolls in a roll coater at a vertically standing state and applying the solder resist composition to both surfaces of the substrate at the same time is adopted as a best application form, the solder resist composition of the conventional technique has a problem in that dropping is caused because of low viscosity.
On the other hand, there has been known a flip chip mounting method wherein solder bumps are arranged on the pads formed on the wiring substrate and connected to an IC chip as a conventional technique suitable for the high densification of mounting components.
This flip chip mounting is a technique wherein conductor circuits including pads are formed on the surface of the printed wiring substrate to be mounted and a solder is fed to resist-opening portions through the solder resist to form solder bumps on the pads and then the solder bumps are subjected to a reflowing treatment to conduct electrical connection between the solder bump and IC chip (electronic component).
However, when the flip chip mounting is applied to an additive type printed circuit board obtained by directly forming a conductor circuit on an insulating resin layer through a permanent resist, there is a problem that the solder bump is apt to be peeled off from the pad surface in the heat cycle.
Moreover, the Applicant has already proposed a printed circuit board having an excellent adhesion property between solder resist layer and metal pad as a technique capable of preventing the peeling of the solder resist layer (see Japanese Patent Application No. 7-68656). Such a proposed technique is a method of forming a solder body through electroless plating wherein a surface of a metal pad is roughened and a copper layer is formed thereon and then substitution plating of copper with tin or tin unhomogeneous plating and further substitution plating of lead are carried out thereon.
However, this technique has problems in that it takes a long time and a high cost and the mass productivity is poor.
Furthermore, the surface of the copper layer is liable to be oxidized and it is difficult to maintain a pure metal surface. If the surface is oxidized, the wettability to solder is poor and hence it is difficult to form solder by a method suitable for mass production such as solder transferring method, solder printing method or the like.